It might be that the 22x8 gives you a lighter load hence easier to spool up and a higher RPM. Of course something like a 23x6 (if it exists and would work) would be better since you do not need the extra pitch for speed. The so called 23x6 would give a lighter load and a bigger diameter assists in giving thrust. But I thought its 22x10 or 23x8 for a 50cc engine ...